Geometrical Cross Sections of Dust Aggregates and a Compression Model for Aggregate Collisions

TL;DR

This paper investigates how dust aggregate geometrical cross sections evolve during collisions, refining a compression model to better predict aggregate growth and properties in protoplanetary disks.

Contribution

It refines a previous compression model to accurately describe the evolution of cross sections and gyration radii of dust aggregates during collisions, including non-equal-mass cases.

Findings

Cross sections decrease with compression and are related to gyration radii.

The refined model accurately reproduces numerical results of aggregate collisions.

Oblique collisions may further inhibit aggregate compression.

Abstract

Geometrical cross sections of dust aggregates determine their coupling with disk gas, which governs their motions in protoplanetary disks. Collisional outcomes also depend on geometrical cross sections of initial aggregates. In the previous paper, we performed three-dimensional N-body simulations of sequential collisions of aggregates composed of a number of sub-micron-sized icy particles and examined radii of gyration (and bulk densities) of the obtained aggregates. We showed that collisional compression of aggregates is not efficient and that aggregates remain fluffy. In the present study, we examine geometrical cross sections of the aggregates. Their cross sections decreases due to the compression as well as their gyration radii. It is found that a relation between the cross section and the gyration radius proposed by Okuzumi et al. is valid for the compressed aggregates. We also refine the compression model proposed in our previous paper. The refined model enables us to calculate the evolution of both gyration radii and cross sections of growing aggregates and reproduces well our numerical results of sequential aggregate collisions. The refined model can describe non-equal-mass collisions as well as equal-mass case. Although we do not take into account oblique collisions in the present study, oblique collisions would further hinder compression of aggregates.